The present invention generally relates to a continuously variable transmission, and more specifically to an apparatus and method for adaptively shifting between ranges in a continuously variable transmission.
Many work machines, particularly earth working machines, use a continuously variable transmission to drive traction wheels or tracks which propel the work machine. Typically, a hydrostatic transmission, consisting of a variable speed hydraulic pump and a hydraulic motor, provides a continuously variable speed output to the wheels or tracks of the work machine. In particular, the speed output can be continuously varied by controlling the displacement of the hydraulic pump.
In order to operate over a wide range of operating conditions, the engine torque is split between the hydrostatic transmission and a mechanical transmission. The mechanical transmission has a number of transmission ranges corresponding to different operating speeds of the work machine. The combination of the hydrostatic transmission and the mechanical transmission allows a continuously variable transmission to operate over a wider range of speeds than is possible using the hydrostatic transmission alone. Many continuously variable transmissions wait to shift when the relative speed of the oncoming clutch is zero. However, depending on the load, this shift point will feel noncontinuous as the machine will speed up or slow down due to the shift.
In order to shift between ranges a shift point must be determined which provides a smooth continuous output speed, regardless of the load on the transmission. By sensing speed of the hydraulic motor, output speed, and displacement of the variable displacement pump, it is possible to determine the amount of adjustment to the shift point required to achieve a shift which produces a smooth continuously variable output. However, a drawback to determining shift points in this manner is that such calculations are complex and require the reading of several sensors, which may fail during operation of the transmission. Therefore, it is desirable to adaptively determine shift points using a minimum amount of sensed inputs.
What is needed therefore is a method and apparatus for adaptively shifting between ranges in a continuously variable transmission which overcomes the above-mentioned drawbacks.
In accordance with a first aspect of the present invention, there is provided a transmission assembly driven by an engine. The transmission assembly includes a hydrostatic transmission having a variable displacement hydraulic pump and a hydraulic motor, a mechanical transmission coupled to the hydrostatic transmission for selecting a first gear range and a second gear range, and a controller for receiving operator inputs and generating a displacement command operable to control a displacement of the variable displacement pump. The controller determines an actual displacement. The controller determines a displacement shift level from the actual displacement. The controller executes a shift from the first gear range to the second gear range when the actual displacement reaches the displacement shift level.
In accordance with a second aspect of the present invention, there is provided a method of controlling a transmission having a hydrostatic transmission with a variable displacement hydraulic pump and a hydraulic motor, a mechanical transmission coupled to the hydrostatic transmission for selecting a first gear range and a second gear range, and a controller for receiving operator inputs and generating a displacement command operable to control a displacement of the variable displacement pump. The method includes the step of determining the actual displacement. The method further includes the step of determining a displacement shift level from the actual displacement with the controller. The method yet further includes the step of executing a shift from the first gear range to the second gear range when the actual displacement reaches the displacement shift level.